In recent years, as the power supply for portable devices such as mobile phones and laptop computers, electric vehicles, and the like, attention has been focused on nonaqueous electrolyte secondary batteries having high energy density, little self-discharge, and excellent cycle performance, such as lithium secondary batteries. Nowadays, the mainstream lithium secondary batteries are small consumer batteries, mainly including mobile phone batteries having a capacity of 2 Ah or less. A large number of proposals have been made as positive active materials for lithium secondary batteries, and the most commonly known are lithium-containing transition metal oxides having an redox potential of about 4 V, whose basic structure is lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4) with a spinel structure, etc. In particular, lithium cobalt oxide has excellent charge-discharge characteristics and energy density, and thus has been widely adopted as a positive active material for small-capacity lithium secondary batteries having a battery capacity of up to 2 Ah.
However, in consideration of the future development of nonaqueous electrolyte batteries into middle-sized and large-sized batteries, particularly those for industrial applications where a huge market is expected, there is a need for a positive active material having higher capacity and higher safety and higher storage performance.
Accordingly, recently, as a positive active material having high capacity and safety, a lithium-containing phosphate having a high proportion of lithium per formula unit, such as Li3V2(PO4)3, has been proposed (see Patent Document 1).
The following invention is also known: “a secondary battery containing as an active material a material represented by MaNbXc (1) [in formula (1), M is selected from H, Li, Na, Mg, Al, K, and Ca, N represents at least one member selected from transition metals, Al, and Cu, X represents a polyanion, a represents 0 to 5, b represents 1 to 2, and c represents 1 to 3]” (see Patent Document 2). Patent Document 2 states “in formula (1), X is preferably at lease one polyanion selected from SiO4, PO4, SO4, MoO4, WO4, BO4, and BO3, and more preferably PO4 or MoO4” (paragraph [0023]). However, examples of the positive active material shown in the Description are “LiFePO4, LiCoPO4, LiNa2PO4, Li3V2(PO4)3, Na3V2(PO4)3, and LiVPO4F, NaVPO4F” (paragraph [0024]). Further, merely “Li3V2(PO4)3, Na3V2(PO4)3, and LiVPO4F” are shown in the Examples (paragraph [0046], Table 1). Thus, the substitution a part of PO4 in Li3V2(PO4)3 with BO3 is not mentioned.
According to the invention described in Patent Document 2, “an object is to provide a secondary battery using a liquid electrolyte with excellent storage characteristics” (paragraph [0007]), and “at the same time, it has been found that when an active material having a so-called NASICON structure is used as a material for the positive electrode and the negative electrode, a secondary battery containing an ionic liquid as the electrolyte with excellent storage characteristics and safety can be provided” (paragraph [0008]). However, it does not suggest that storage characteristics are improved by the substitution a part of PO4 in Li3V2(PO4)3 with BO3.
The following invention is also known: “a method for preparing a composite material containing an electrode active compound of formula AaDdMmZzOoNnFf (wherein A is an alkali metal, D is selected from alkaline earth metals and elements of Group III of the element periodic table (except for B), M is a transition metal or a transition metal mixture, Z is a nonmetal selected from S, Se, P, As, Si, Ge, Sn, and B, O is oxygen, N is nitrogen, F is fluorine, and a, d, m, z, o, n, and f are each a real number of 0 or more and selected to ensure electrical neutrality) and an electrically conductive compound like carbon, the method including thermally decomposing a homogeneously mixed precursor within a short period of time to produce the composite material, the precursor containing all the elements A, D, M, Z, O, N, and F forming the electrode activity compound and at least one organic compound and/or organometallic compound” (see Patent Document 3). Patent Document 3 states “A is selected from Li, Na, K, and mixtures thereof” (claim 3) and “M is selected from Fe, Ni, Co, Mn, V, Mo, Nb, W, Ti, and mixtures thereof” (claim 5). However, an electrode active compound specifically mentioned is “a lithium insertion compound or a sodium insertion compound, such as LiFePO4, LiFeBO3, or NaFeBO3” (claim 6). Thus, an electrode active compound with PO4 in Li3V2(PO4)3 or Li3V2(PO4)3 being partially substituted with BO3 is nowhere mentioned.
Patent Document 3 states “there also is a need for a method for preparing a composite material that provides a high-purity final product with completely controlled, homogeneous morphology. This composite material shows excellent electrochemical kinetics, and can be used at high charge/discharge rate” (paragraph [0035]). However, storage performance is nowhere suggested.
Further, the following invention is known: “a nonaqueous electrolyte secondary battery containing, as a positive active material, a compound represented by composition formula Li1+aFeP1−xMxO4−b (M: at least one element selected from trivalent elements, 0<x<1, 0≦a≦2x, and 0≦b≦x, with the proviso that x, a, and b are selected to allow a compound represented by the composition formula to maintain electrical neutrality); as a negative active material, a substance capable of reversibly inserting/extracting or absorbing/releasing lithium, other alkali metals, or ions thereof; and, as an electrolytic substance, a substance that is chemically stable to a positive active material and a negative active material and capable of moving the ions thereof to undergo an electrochemical reaction (see Patent Document 4). Patent Document 4 states “the positive active material is a compound wherein M is at least one of B and Al” (claim 2). It also shows that the discharge capacity is greatly increased by the substitution a part of PO4 in LiFePO4 with BO3 (paragraph [0040], Table 1). However, the substitution a part of PO4 in Li3V2(PO4)3 with BO3 is nowhere suggested, and there is no suggestion about storage performance either.
Patent Document 5 discloses an electrode active material (claim 4) containing “a compound represented by the following formula (1):LiMP1−xAxO4  Formula (1)(wherein M is a transition metal, A is an element having an oxidation number of +4 or less, and 0<X<1)” (claim 1). Patent Document 5 also states “in the formula, M is at least one transition metal selected from the group consisting of Fe, Co, Mn, Ni, V, Cu, and Ti” (claim 2) and “in the formula, A is an element selected from the group consisting of Ti (4+), Al (3+), B (3+), Zr (4+), Sn (4+), V (4+), Pb (4+), and Ge (4+)” (claim 3). However, it does not specifically states that M is V and A is B (3+), and there is no suggestion about storage performance either.
Patent Document 6 discloses an improvement of the charge capacity and cycle life of a cathode material of a battery having a polyanionic powder of lithium metal, and states “the polyanionic powder of lithium metal has a polyanion containing boron, phosphorus, silicon, aluminum, sulfur, fluorine, chlorine, or a combination thereof” (claim 2) and “the polyanion contains BO33−, PO43−, AlO33−, AsCl4−, AsO33−, SiO33−, SO42−, BO3−, AlO2−, SiO32−, SO42−, or a combination thereof” (claim 3). However, merely a lithium vanadium phosphate powder (Example 2) is specifically shown. The substitution a part of PO43− with BO33− is nowhere suggested, and there is no suggestion about storage performance either.